Process for defoaming acid gas scrubbing solutions and defoaming solutions

ABSTRACT

The foam in acid gas scrubbing solutions created during an acid gas scrubbing process is reduced or eliminated by the addition of certain polyoxyethylene polyoxypropylene block copolymers as defoaming agents. The defoaming agents are particularly effective when the acid gas scrubbing solution contains an amine having a large hydrophobic moiety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a process for reducing or eliminatingfoam in an acid gas scrubbing process stream by the addition of certainpolyoxyethylene polyoxypropylene block copolymers. The present inventionalso includes acid gas scrubbing solutions containing these blockcopolymers.

2. Description of the Prior Art

Acid gas scrubbing processes are of considerable industrial importance.Most acid gas scrubbing processes involve contacting a gas stream withan aqueous alkaline solution and absorbing the acid gas, principallycarbon dioxide and hydrogen sulfide, into the solution and regeneratingthe solution in a separate stage. The regenerated solution is thenrecycled to the absorption stage. One of the problems encountered inacid gas scrubbing processes is excess foaming, i.e., aqueous alkalinesolutions have a natural inclination to foam, and the passing of a gasinto a foamable solution is classic to the production of a foam.Consequently, resort to the use of antifoaming agents as additives tothe aqueous acid gas scrubbing solutions is a common practice in theart.

There are many commercially available antifoaming agents. Theseantifoaming agents find a plurality of uses. However, the selection ofan antifoaming agent in acid gas scrubbing processes is dependent uponseveral variables unique to acid gas scrubbing processes. Theantifoaming agents of the present invention are particularly suited foracid gas scrubbing processes.

The following U.S. patents were considered in connection with thepresent invention:

    ______________________________________                                        U.S. Pat. Nos.                                                                ______________________________________                                        2,176,441        3,848,057                                                    2,608,462        3,862,243                                                    2,674,619        3,943,227                                                    2,712,978        4,002,721                                                    2,946,652        4,042,528                                                    3,071,433        4,094,957                                                    3,275,403        4,100,257                                                    3,642,430        4,101,633                                                    ______________________________________                                    

None of these patents disclose, teach or suggest the use ofpolyoxyethylene polyoxypropylene block copolymers as antifoaming agentsin acid gas scrubbing processes. Many of these patents teach acid gasscrubbing processes and the general use of conventional antifoamingagents in their respective processes. Of particular interest are U.S.Pat. Nos. 4,094,957; 4,100,257; and 4,101,633, in addition to U.S. Pat.Nos. 4,112,050; 4,112,051; and 4,112,052, the disclosures of which areincorporated herein by reference. These patents and patent applicationsteach and disclose the use of sterically hindered amines, alone, or incombination with other acid gas scrubbing compositions in acid gasscrubbing processes. These patents and patent applications point outthat there are three principal types of acid gas processes, i.e., theaqueous amine process (amine and water constitute the acid gas scrubbingsolution), the "hot potash" process (an inorganic alkaline material suchas potassium carbonate activated by at least one amine) and the organicsolvent process (an organic solvent such as sulfolane used incombination with an amine). Foaming problems are especially troublesomein the "hot potash" type processes such as described and claimed in U.S.Pat. No. 4,094,957 and U.S. Pat. No. 4,112,050.

The prior art has included disclosures of certain block copolymers ofoxyalkylene moieties as antifoamers for acid gas scrubbing processes.For example, U.S. Pat. No. 3,862,243 (in the list above) disclosespolyoxyethylene polyoxybutylene block copolymers of the formulaR(OEt)_(n) (OBu)_(m) OH where R is an alkyl or alkenyl group having 6-22carbon atoms, OEt is oxyethylene, OBu is oxybutylene, n is from 3 to 22and m is from about 3 to 15. It is disclosed in this patent that theseblock copolymers may be used as antifoamers in alkanolamine acid gasscrubbing processes. Also, the technical brochure entitled "TheWonderful World of Pluronic Polyols", published 1973 by BASF WyandotteCorporation, Wyandotte, Michigan 48192 discloses polyoxyethylenepolyoxypropylene block copolymers as antifoamers and in particularreference is made to their use in "gas-treating units".

It has been recently discovered that high foams in amine-carbonatesolutions, where the amine has a large hydrophobic moiety, do notrespond adequately to conventional hydrocarbon- and silicone-basedantifoaming agents when in use in acid gas scrubbing processes. Mostconventional antifoaming agents work by spreading at the air/solutioninterface thereby disrupting the films formed by surface activematerials dissolved in the solution. This mechanism has been found to beineffective in hot amine-carbonate solutions where the amine has a largehydrophobic moiety in addition to its hydrophilic amine groups. Thisproblem is particularly acute in the case of sterically hindered amineactivators in hot carbonate solutions such as disclosed in U.S. Pat. No.4,094,957 and U.S. Pat. No. 4,112,050.

It has now been discovered that foam-breaking of thesehydrophobe-containing amine-carbonate solutions ca be accomplished bylowering the hydrophile-lipophile balance (HLB) of the foam-formingsurfactants (i.e., the hydrophobe-containing amines act as surfactantsin the hot carbonate solutions) by the addition of a low HLB surfactantwhich is itself a low foamer. It has been found that one class of lowHLB surfactants useful as a foambreaker in these hydrophobicamine-carbonate solutions are the polyoxyethylene polyoxpropylene blockcopolymers described herein. This discovery is unexpected, since the useof these block copolymers in aqueous amine acid gas scrubbing solutions(i.e., those solutions where the amine is used as the sole absorbent)are ineffective in controlling foaming. For example, it has been shownthat the addition of BASF-Wyandotte Pluronic polyol L72 to an aqueoussolution of methyldiethanolamine (MDEA) orN-cyclohexyl-1,3-propanediamine (CHPD) gas treating solution causedexcessive foaming.

SUMMARY OF THE INVENTION

The present invention is directed to an acid gas scrubbing solutioncomprising: (a) an alkaline material comprising a basic alkali metalsalt or alkali metal hydroxide, (b) an activator for said alkalinematerial comprising at least one amine having a hydrophobic moiety, and(c) water, the improvement comprising the addition of a defoaming amountof an antifoaming agent defoamer comprising polyoxyethylenepolyoxypropylene block copolymers of the general formula:

    HO(C.sub.2 H.sub.4 O).sub.a (C.sub.3 H.sub.6 O).sub.b (C.sub.2 H.sub.4 O).sub.c H

wherein a and c are integers such that the hydrophile portionrepresented by (C₂ H₄ O) constitutes from about 10 to about 30, andpreferably from about 15 to about 25 weight percent of the copolymer,and b is an integer such that the hydrophobe represented by (C₃ H₆ O)has a molecular weight from about 1000 to about 2150, preferably fromabout 1200 to about 2100, and most preferably from about 1750 to about2050. The total molecular weight of the copolymers of use in theinvention should be such that the block copolymer is in liquid form andwater soluble.

The amount of the polyoxyethylene polyoxypropylene block copolymers inthe acid gas scrubbing solution is not critical in terms of defoamingactivity. Typically, the antifoaming agent block copolymers will bepresent in an amount ranging from about 1 to about 1500 parts permillion (ppm), preferably from about 5 to about 50 ppm per weight of thetotal acid gas scrubbing solution.

The hydrophobe of the polyoxyethylene polyoxypropylene block copolymersdescribed herein may be prepared by adding propylene oxide to the twohydroxyl groups of a propylene glycol nucleus. By adding ethylene oxideto the hydrophobe, it is possible to place polyoxyethylene hydrophilicgroups on both ends of the molecule. These hydrophilic polyoxyethylenegroups may be controlled to constitute anywhere from about 10 to about30 weight percent of the final molecule. A more detailed description ofthe preparation of these block copolymers may be found in U.S. Pat. No.2,674,619, the disclosure of which is incorporated herein by reference.Alternatively, the polyoxyethylene polyoxypropylene block copolymers maybe obtained commercially, e.g., they are presently available from BASFWyandotte Corporation and identified as Pluronic (Registered Trademark)polyols L42, L43, L61, L62, L63 and L72.

The present invention is also directed to using the aforesaid acid gasscrubbing solution, which includes the polyoxyethylene polyoxypropyleneblock copolymers, in a process for removing CO₂ from gaseous feedscontaining CO₂.

The term "acid gas" as used herein, includes CO₂ alone or in combinationwith H₂ S, SO₂, SO₃, CS₂, HCN, COS and the oxides and sulfur derivativesof C₁ to C₄ hydrocarbons. The major constituents of the acid gas will becomprised of CO₂ and/or H₂ S.

The term "hydrophobic moiety", which is part of the amine activator, ismeant to include a moiety which itself is not wetted by water. Typicalhydrophobic moieties include alkylene, alkyl, and substituted alkyleneand alkyl groups having three or more carbon atoms. Also included arecycloalkyl groups having three or more carbon atoms.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The antifoaming agent comprising the polyoxyethylene polyoxypropyleneblock copolymers may be added separately or in combination with the acidgas scrubbing solution during the process of removing the acid gasesfrom the gaseous streams. Preferably, the alkaline material will be analkali metal salt or alkali metal hydroxide and most preferably analkali metal salt such as potassium carbonate. The amine activatorcontaining a hydrophobic moiety may be 1,6-hexanediamine, piperidine ortheir derivatives (as disclosed in U.S. Pat. Nos. 3,637,345; 3,793,434;and 3,848,057, the disclosures of which are incorporated herein byreference), sterically hindered amines, i.e., amines wherein at leastone secondary amino group is attached to either a secondary or tertiarycarbon atom or a primary amino group is attached to a tertiary carbonatom, especially preferred, the sterically hindered aminoethers,aminoalcohols, di- and triamines such as disclosed in U.S. Pat. No.4,094,957 and U.S. Pat. No. 4,112,050, the disclosures of which areincorporated herein by reference, and other hydrophobic amineactivators, coactivators and mixtures thereof.

The absorbing solution containing the block copolymer antifoaming agentof the invention will preferably be comprised of a major proportion ofan alkaline material, e.g., alkali metal salts and a minor proportion ofthe amine activator. The salt content of the acid gas scrubbing solutionwill be in the range from about 10 to 40 weight percent, preferably 20to 30 weight percent. The content of the amine activator, preferably asterically hindered amine, in the solution will be in the range fromabout 2 to about 20 weight percent, preferably 5 to about 15 weightpercent and more preferably 5 to about 10 weight percent. As a preferredembodiment, the amine activator will be comprised of a stericallyhindered amine which may be used alone or in combination with otheramines such as diethanolamine and/or aminoacid cosolvents for thesterically hindered amines as disclosed and claimed in U.S. Pat. No.4,094,957. The aminoacid cosolvent will include those aminoacids having4 to 8 carbon atoms which increase the solubility of sterically hinderedamines in alkaline aqueous conditions at elevated temperatures, e.g.,N,N-dimethylglycine, 3-amino-3-methyl butyric acid,2-amino-2-methyl-butyric acid, and pipecolinic acid. The preferredsterically hindered amines include: The cycloaliphatic diamines such asN-cyclohexyl-1,2-ethanediamine, N-cyclohexyl-1,3-propane-diamine,N-cyclohexyl-1,4-butanediamine and N-cyclohexyl-1,5-pentanediamine; thepiperazines such as 2-methylpiperazine and 2,5-dimethylpiperazine andthe diamine, N₁(1,1-dimethyl-2-hydroxyethyl)-2-methyl-1,2-propanediamine. The mostpreferred sterically hindered amines are N-cyclohexyl-1,3-propanediamineand N-cyclohexyl-1,4-butanediamine. The most preferred cosolvent ispipecolinic acid.

Other additives in addition to the antifoaming agent of the inventioncan be included in the acid gas scrubbing solution, e.g., antioxidants,corrosion inhibitors, etc. Also, the acid gas scrubbing solution maycontain mixtures of amines, e.g., the sterically hindered amines may beused in combination with diethanolamine. The antifoaming agent of theinvention may be used in combination with other antifoaming agents.

The contacting of the absorbent mixture containing the antifoaming agentof the invention and the acid gas may take place in any suitablecontacting tower. In such processes, the gaseous mixture from which theacid gases are to be removed may be brought into intimate contact withthe absorbing solution using conventional means, such as a tower packedwith, for example, ceramic rings or with bubble cap plates or sieveplates, or a bubble reactor. The antifoaming agent may be fed into theabsorber alone or with the absorbent mixture and recycled as such.

In a preferred mode of practicing the invention, the absorption step isconducted by feeding the gaseous mixture into the base of the towerwhile fresh absorbing solution is fed into the top. The gaseous mixturefreed largely from acid gases emerges from the top. Preferably, thetemperature of the absorbing solution during the absorption step is inthe range from about 25° to about 200° C., and more preferably from 35°to about 150° C. Pressures may vary widely; acceptable pressures arebetween 5 and 2000 psia, preferably 100 to 1500 psia, and mostpreferably 200 to 1000 psia in the absorber. In the desorber thepressures will range from about 5 to 100 psia. The partial pressure ofthe acid gas, e.g., CO₂ in the feed mixture, will preferably be in therange from about 0.1 to about 500 psia, and more preferably in the rangefrom about 1 to about 400 psia. The contacting takes place underconditions such that the acid gas, e.g., CO₂, is absorbed by thesolution. Generally, the countercurrent contacting to remove the acidgas will last for a period of from 0.1 to 60 minutes, preferably 1 to 5minutes. During absorption the solution is preferably maintained in asingle phase, e.g., by action of an aminoacid cosolvent in the event arelatively insoluble sterically hindered amine is used as the K₂ CO₃activator. The aminoacid cosolvent also aids in foam reduction.

The absorbing solution comprising the aqueous mixture containing thealkaline metal salt and preferably the sterically hindered amine andaminoacid which is saturated or partially saturated with gases, such asCO₂ and H₂ S may be regenerated so that it may be recycled back to theabsorber. The regeneration should also take place in a single liquidphase. Therefore, the presence of an aminoacid cosolvent provides anadvantage in this part of the overall acid gas scrubbing process. Theregeneration or desorption is accomplished by conventional means, suchas pressure reduction, which causes the acid gases to flash off or bypassing the solution into a tower of similar construction to that usedin the absorption step, at or near the top of the tower, and passing aninert gas such as air or nitrogen or preferably steam up the tower. Thetemperature of the solution during the regeneration step may be the sameas used in the absorbing step, i.e., 25° to about 200° C. and preferably35° to about 150° C. The absorbing solution, after being cleansed of atleast a portion of the acid bodies, may be recycled back to theabsorbing tower. Makeup absorbent may be added as needed. Single phaseis maintained during desorption by controlling the acid gas, e.g., CO₂,level so that it does not fall into the region where two liquid phasesform. This, of course, following the practice of the present inventionis facilitated by the use of the aminoacid cosolvent in combination withthe sterically hindered amine.

For example, during desorption, the CO₂ -rich solution from the highpressure absorber is sent first to a flash chamber where steam and someCO₂ are flashed from solution at low pressure. The amount of CO₂ flashedoff will, in general, be about 35 to 40% of the net CO₂ recovered in theflash and stripper. This is increased somewhat, e.g., to 40 to 50%, withthe high desorption rate promoter system owing to a closer approach toequilibrium in the flash. Solution from the flash drum is then steamstripped in the packed or plate tower, stripping steam having beengenerated in the reboiler in the base of the stripper. Pressure in theflash drum and stripper is usually 16 to about 100 psia, preferably 16to about 30 psia, and the temperature is in the range from about 25° toabout 200° C., preferably 35° to about 150° C., and more preferably 100°to about 140° C. Stripper and flash temperatures will, of course, dependon stripper pressure, thus at about 16 to about 25 psia stripperpressures, the temperature will preferably be about 100° to about 140°C. during desorption. Single phase can be maintained during desorptionby regulating the amount of acid gas, e.g., CO₂, recovered.

The gaseous mixture freed largely from acid gases emerging from the topof the absorber may then be further scrubbed in a caustic scrubber towerby countercurrent contact with aqueous alkali metal hydroxide, (e.g., anaqueous solution containing about 1-10% by weight NaOH). If foamingdevelops in the caustic scrubber, the polyoxyethylene polyoxypropyleneblock copolymer defoamer of the present invention may be added to thecaustic solution to reduce foaming in the scrubber. The gases enteringinto the caustic scrubber may also contain residual amine activator, thepresence of which may enhance foaming in the presence of the sodiumhydroxide. Following caustic scrubbing, the gases may be passedcounter-currently through water as a final cleanup.

The invention is illustrated further by the following examples which,however, are not to be taken as limiting in any respect. All parts andpercentages, unless expressly stated to be otherwise, are by weight.

EXAMPLE 1

This example shows the defoaming effect of the polyoxyethylenepolyoxypropylene block copolymers of the present invention as comparedagainst other polyoxyethylene polyoxypropylene block copolymers.

(a) The following test is used to quickly determine potential candidateantifoaming agents in hydrophobe-containing amine-carbonate acid gasscrubbing solutions. A predetermined amount of a potential defoamer (upto 1000 ppm) is weighed into a 250 ml erlenmeyer flask containing 100 g.of N-cyclohexyl-1,3-propanediamine (CHPD)-carbonate (K₂ CO₃) solution(e.g., 7.4% CHPD, 6.1% pipecolinic acid and 25% K₂ CO₃). A watchglass isplaced on top of the flask (to keep down the evaporation), then theflask is stirred and heated to 80°-90° C. The stirring is stopped, and astainless-steel sparger (60 mesh) through which about 1 liter/minute ofnitrogen is flowing, is introduced into the solution. If foam is presentin large quantities, the solution is discarded. If no foam or a slightfoam appears and it persists as a low foam, this defoamer is then heldfor further testing as shown in section (b). The results of thisscreening test are shown in the following table.

                  TABLE 1                                                         ______________________________________                                        Antifoam Block                                                                            Mol. Wt. of                                                                              Wt. % of   Results of                                  Copolymer   Hydrophobe Hydrophile Preliminary                                 (Pluronic®  polyol)                                                                   (C.sub.3 H.sub.6 O)                                                                      (C.sub.2 H.sub.4 O)                                                                      Screening                                   Code Nos.   (average)  (average)  Tests.sup.(a)                               ______________________________________                                        L31         950        10         -                                           L42         1200       20         ±                                        L43         1200       30         +                                           L44         1200       40         +                                           L61         1750       10         +                                           L62         1750       20         +                                           L63         1750       30         +                                           L64         1750       40         -                                           L72         2050       20         +                                           L81         2250       10         +                                           L92         2750       20         +                                           L101        3250       10         +                                           P84         2250       40         +                                           P94         2750       40         -                                           P104        3250       40         +                                           ______________________________________                                         .sup.(a) In each of the tests approximately 1000 ppm to 3000 ppm of the       block copolymer is added to the test solution. The symbol - indicates the     solution foamed and the block copolymer did not pass the initial screenin     test. The symbol + indicates the test solution did not foam and the block     copolymer was selected for further screening as shown below. The symbol       ± indicates the test solution foamed slightly.                        

(b) As a further screening of the antifoaming agents, the blockcopolymers which were indicated positive, +, in the preliminaryscreening tests are subjected to the following quantitative tests. Fivehundred grams of the amine-carbonate solution (e.g., 7.4% CHPD, 6.1%pipecolinic acid and 25% K₂ CO₃) is put into a cyclindrical graduated 4liter jacketed gas trap apparatus. A circulating oil bath surrounds thecyclinder to maintain the temperature of the unit at 80° C. A gassparger is inserted through an adapter with a reflux condenser on theside arm to insure the integrity of the starting solution. Theamine-carbonate solution put in the apparatus is then allowed toequilibrate at the 80° C. level. A calibrated flow (1/2 to 4liters/min.) of nitrogen gas is introduced into the solution containinga designated amount of the candidate defoamer, and the foam is allowedto stabilize. The height of the foam is then measured from the base ofthe foam to the top. With this number recorded, the gas is then stoppedabruptly and the foam allowed to settle and the time required ismeasured. The foam height can be plotted as a function of gas rate.

The steps above are repeated with increasing amounts of defoamer. Theresults of this screening test are shown in Table II.

                  TABLE II                                                        ______________________________________                                        Antifoam    Amount of Foam Height, ml                                         (Pluronic™                                                                             Antifoam, Gas Rate, Liters/min.                                   Polyol Code Nos.)                                                                         ppm       1      2    3    4                                      ______________________________________                                        Control     --        150    500  900  1800                                   L42         10        250    950  1300 1300                                   L42         20        350    1050 1300 1350                                   L42         50        200    450  600  700                                    L42         100       200    300  400  500                                    L42         1000      150    200  150  200                                    L42         1500      25     50   50   25                                     L43         10        350    700  1100 1500                                   L43         20        300    650  950  1200                                   L43         50        250    400  550  600                                    L43         100       100    200  200  200                                    L43         200       30     60   60   90                                     L44         10        600    1600 2000 2100                                   L44         20        350    1400 1400 1700                                   L44         50        50     200  300  450                                    L44         100       50     200  300  500                                    L61         10        500    900  900  1000                                   L61         20        500    800  900  1000                                   L61         50        500    800  800  900                                    L61         100       500    800  800  800                                    L61         1000      500    750  750  700                                    L61         1500      400    700  700  700                                    L62         10        250    650  800  700                                    L62         20        350    550  650  700                                    L62         50        200    450  550  600                                    L62         100       200    450  650  550                                    L62         1000      150    400  450  450                                    L62         1500      100    350  400  350                                    L63         10        100    350  300  450                                    L63         20        50     300  450  500                                    L63         50        50     250  300  300                                    L63         100       50     200  200  200                                    L72         10        300    400  500  550                                    L72         20        200    400  400  500                                    L72         50        150    350  400  450                                    L72         100       150    350  400  450                                    L72         1000      150    300  300  300                                    L72         1500      125    200  300  300                                    L81         1500      340    210  20   340                                                                           (charred)                              Control     --        150    500  900  1800                                   L92         10        300    900  900  1000                                   L92         20        450    900  --   --                                     L92         50        400    800  --   --                                     L92         100       No change                                               L101        10        450    800  900  1000                                   L101        20        450    900  1000 1000                                   L101        50        450    900  1000 1000                                   L101        100       450    900  1000 1000                                   L101        1000      250    750  750  800                                    L101        1500      200    500  650  700                                    P84         10        300    800  1100 1350                                   P84         20        400    900  1100 1400                                   P84         50        300    900  1000 1250                                   P84         100       350    800  1100 1300                                   P104        10        350    850  (Defoamer                                                                     charred) Ex-                                                                  periment Abor-                                                                ted)                                        ______________________________________                                    

Based on the tests shown in Table II, the antifoam block copolymersidentified as Pluronic™ polyol L81, L92, L101, P84, and P104 are deemedinferior to Pluronic™ polyol L42, L43, L61, L62, L63 and L72. The latterblock copolymers were able to reduce the foam height by at least a halfthe level of the control sample at a gas flow rate of 4 L/min when 100ppm of the antifoamer is added to the acid gas scrubbing solution.

(c) An important aspect of a good antifoaming agent for use in acid gasscrubbing processes is its ability to retain its integrity duringprolonged use. Two of the block copolymers, Pluronic™ polyol L42 and L72are tested for their ability to retain their integrity by adding theblock copolymer to a CHPD/pipecolinic acid/K₂ CO₃ acid gas scrubbingsolution (e.g., 7.4% CHPD, 6.1% pipecolinic acid and 25% K₂ CO₃). Theacid gas scrubbing solution containing the antifoaming agent is placedin a double ended 500 ml stainless steel cylindrical bomb. One end ofthe bomb is capped with a swagelock fitting, and the acid gas scrubbingsolution with added antifoaming agent is poured in the other end. Theopen end of the bomb is then sealed with a 2000 Kpc safety, and theentire bomb is placed in a 120° C. oven and allowed to remain there fora 500-hour period. The solution is removed, cooled, and put into the gassparging apparatus described in 1(b) above. The results of this test areshown in Table III.

                  TABLE III                                                       ______________________________________                                        Antifoam                                                                      Block Copolymer                                                                            Amount of   Foam Height, ml                                      (Pluronic®                                                                             Antifoam,   Gas Rate, Liters/min                                 polyol Code Nos.)                                                                          ppm         1      2    3    4                                   ______________________________________                                        L42          100         150    230  300  400                                 L72           50          30    150  150  200                                 ______________________________________                                    

The results in Table III show that the polyoxyethylene polyoxypropyleneblock copolymers having 20 weight percent of the hydrophile in thecopolymer and having a hydrophobe having an average molecular weight ofabout 1200 to about 2050 are able to withstand rather a severe agingtest and retain their ability to act as a defoamer for the foamproducing amine-carbonate acid gas scrubbing solution. The aging testcan be repeated for additional 500-hour sequences a number of times toascertain the durability of antifoamers.

EXAMPLE 2

An add-on test is simulated wherein a small amount ofN-cyclohexyl-1,3-propanediamine (CHPD) and cosolvent, pipecolinic acidare added to a diethanolamine (DEA) acid gas scrubbing solution. In oneexperiment there is no antifoaming agent present (except for theantifoaming action of the DEA), and in the other 10 ppm of Pluronic™polyol L72 is added. Each of the respective acid gas scrubbing solutionsare placed in the apparatus described in Example 1(b) above. Acalibrated flow (1/2 to 4 liters/minute) of nitrogen gas is introducedinto the solution, and the foam is allowed to stabilize. The height ofthe foam is then measured from the base of the foam to the top. Withthis number recorded the gas is then stopped abruptly, and the foam isallowed to settle, and the time required is measured. The acid gasscrubbing solution (Solution A) without the antifoaming agent has thefollowing composition:

    ______________________________________                                        Acid Gas Scrubbing Solution A                                                 ______________________________________                                         31.5 g.        diethanolamine (DEA)                                          187.5 g.        K.sub.2 CO.sub.3                                              36.9 g.         H.sub.3 BO.sub.3                                              3.7 g.          CHPD                                                          1.8 g.          pipecolinic acid                                              480.0 g.        water                                                         ______________________________________                                    

The test solution containing the antifoaming agent solution is the sameas described above except that it contains 10 ppm of the Pluronic™polyol L72, the most preferred nonionic antifoaming agent for the CHPDamine K₂ CO₃ activator. The results of the tests using the abovedescribed acid gas scrubbing solutions are shown below.

                  TABLE IV                                                        ______________________________________                                                          Foam Height, ml                                             Acid Gas Scrubbing                                                                              Gas Rate, Liters/min                                        Solution Tested   0.5    1      2    3    4                                   ______________________________________                                        Solution A (no anti-                                                          foam)             10     100    350  500  700                                 Solution B (10 ppm                                                            Pluronic®  polyol                                                         L72)              10      50    100  125  125                                 ______________________________________                                    

The results in Table IV show that the antifoaming agent of the inventionconsiderably reduces the foam in the acid gas scrubbing solution.

EXAMPLE 3

An example of the process of removing CO₂ from a gaseous streamcontaining CO₂ using an amine activated potassium carbonate solutionwith an antifoaming agent of the present invention is carried out in agas treating unit described below. The gas treating unit includes anabsorber, a regenerator, a flash drum situated and connected between theabsorber and the regenerator and a reboiler which is connecteddownstream to the regenerator. The absorber is a cylindrical vesselhaving an inside nominal diameter of about 6 inches, and the regeneratoris a cyclindrical vessel having an inside nominal diameter of about 8inches. Both of the vessels contain 5/8" stainless steel pull rings andhave a bed height of about 5 feet.

The acid gas treating unit is operated by continuously feeding a typicalgaseous feed (i.e., a gas containing N₂, H₂, CO₂, H₂ S, and may containa trace of CH₄) into the absorber into the side of the lower portion ofthe absorber at the rate of 107-116 standard cubic feet per minute(SCF/M). The gaseous feed typically contains 13-15 mole % CO₂ and about0.5 to 0.6 mole % H₂ S. The acid gas scrubbing solution (e.g., thesolution is comprised of 30 weight % K₂ CO₃, 7.4 weight %N-cyclohexyl-1,3-propanediamine and 3.0 weight % pipecolinic acid) isfed into the side of the top portion of the absorber at a continuouscirculation rate ranging from about 2.5 to about 4 gallons per minute.The absorber is maintained at a temperature ranging from about 199°F.-205° F. and at a pressure ranging from about 190-195 psia. Thetreated gas exits the top of the absorber and typically contains (in thetest examples) 3.4-5.6 mole % CO₂ and less than 400 parts per million H₂S. The CO₂ rich spent acid gas scrubbing solution exits the bottom ofthe absorber and is fed to the flash drum by a transport pipe whichincludes a steam source. The CO₂ rich solution is freed of some of theacid gases in the flash drum. The gases exiting the flash drum arecombined with the gases leaving the regenerator by suitable transportlines. Means are provided to analyze the CO₂ gas from these transportlines. The partially regenerated acid gas scrubbing solution iscontinuously fed into the side of the top portion of the regenerator ata rate of about 2.5 to about 4 gallons per minute.

Steam is fed into the side of the lower portion of the regenerator toaffect countercurrent contact with the down flowing solution. The acidgases are carried out the top of the regenerator and combined by acommon conduit with the acid gases vented from the flash drum. Theregenerator is maintained at a temperature ranging from about 235°-245°F. and at pressures ranging from about 5-15 psia (preferably atatmospheric pressures). The CO₂ lean acid gas scrubbing solution exitsthe bottom of the regenerator by a transport line to a reboileroperating at a steam rate of about 176 pounds/hour. Used steam from thereboiler is passed into the lower portion of the regenerator by asuitable transport conduit to aid in the heat balance of the overallsystem. This steam may carry a portion of the lean acid gas scrubbingsolution to the regenerator. The regenerated acid gas scrubbing solutionis then recycled to the top portion of the absorber by a transport linewith heat exchanger means to adjust the temperature of the solution(i.e., cold water may be used to cool the solution, whereas added steammay be used to elevate the temperature of the solution). Duringoperation the entire system is kept in balance. As the chemicalabsorbents, e.g., K₂ CO₃, CHPD and pipecolinic acid are depleted, freshchemicals can be added as needed to maintain a constant level ofabsorption.

In one experimental run (Run No. 1832) using the gas treating unitdescribed above, a gaseous feed containing 13.8 mole % CO₂ is fed intothe absorber at a feed rate of 107 SCF/M while the above described acidgas absorbing solution is fed counter-currently at a continuouscirculation rate of 2.49 GPM. The gas leaving the top of the absorbercontains 3.7 mole % CO₂. Steam is fed into the regenerator at the rateof 1.40 pounds per gallon. The lean conversion is determined as 0.209(CO₂), and the amount of CO₂ absorbed is 11.3 SCF/M. The pressure dropin the absorber is 1.55 inches of water (theoretical is 0.82 inches ofwater). The pressure drop in the regenerator is 4.13 inches of water(theoretical is 2.30). Considerable foam appeared in the regenerator,and some foam appeared in the absorber.

The above experiment was repeated a number of times using the same acidgas absorbing solution with the addition of 5 parts per million ofPluronic.sup.™ polyol L72 to control the foam. The addition of theantifoaming agent of the invention reduced the foam considerably andpermitted the circulation rate of the acid gas scrubbing solution toincrease from 2.49 GPM to 3.56 GPM (Run No. 1834). Of particularsignificance, the pressure drop in the regenerator is reduced from 4.13inches of water (in the case of no antifoaming agent) to a level of 1.05inches of water (Run No. 1834) in the case where 5 ppm of theantifoaming agent is added.

In full scale plant operations there is often a caustic scrubberconnected to the top of the absorber which is used to remove the lasttraces of acid gases. The caustic scrubber also functions as a trap forany amines that are carried into the conduit leading to the scrubber bythe treated gas. It has been found that when the amine activator, e.g.,CHPD, is carried into the caustic scrubber by the treated gas orotherwise (e.g., volatilization) the antifoaming agent of the invention(particularly Pluronic™ polyol L72) is effective in reducing oreliminating the foam in this unit.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as coming within known or customary practice in theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth, and as falling within thescope of the invention.

What is claimed is:
 1. In a process for removing CO₂ from gaseous feedscontaining CO₂, which comprises:(1) contacting said gaseous feeds withan aqueous scrubbing solution comprising: (a) an alkaline materialcomprising a basic alkali metal salt or alkali metal hydroxide, (b) anactivator for said alkaline material comprising at least one amineactivator for said alkaline material, (c) water and (d) at least oneantifoaming agent; and (2) regenerating said aqueous scrubbing solutionat conditions whereby CO₂ is desorbed from said aqueous scrubbingsolution, the improvement which comprises providing at least oneantifoaming agent comprising polyoxyethylene polyoxypropylene blockcopolymers of the general formula:

    HO(C.sub.2 H.sub.4 O).sub.a (C.sub.3 H.sub.6 O).sub.b (C.sub.2 H.sub.4 O).sub.c H

wherein a and c are integers such that the hydrophile portionrepresented by (C₂ H₄ O) constitutes from about 10 to about 30 weightpercent of the block copolymer and b is an integer such that thehydrophobe represented by (C₃ H₆ O) has a molecular weight from about1000 to about
 2150. 2. The process of claim 1 wherein a and c of saidblock copolymer are integers such that the hydrophile portionrepresented by (C₂ H₄ O) constitutes from about 15 to about 25 weightpercent of the block copolymer and b is an integer such that thehydrophobe represented by (C₃ H₆ O) has a molecular weight ranging fromabout 1200 to about
 2100. 3. The process of claim 2 wherein b of theblock copolymer is such that the hydrophobe portion represented by (C₃H₆ O) has a molecular weight ranging from about 1750 to about
 2050. 4.The process of claim 1 wherein at least one of the amine activators is asterically hindered amine defined as a compound containing at least onesecondary amine group attached to either a secondary or tertiary carbonatom or a primary amino group attached to a tertiary carbon atom.
 5. Theprocess of claim 4 wherein said sterically hindered amine is selectedfrom the group consisting of aminoethers, aminoalcohols, di- andtriamines.
 6. The process of claim 4 wherein the sterically hinderedamine is a diamine.
 7. The process of claim 6 wherein the diamine is acycloaliphatic diamine.
 8. The process of claim 6 wherein the diamine isselected from the group consisting of N-cyclohexyl-1,2-ethanediamine,N-cyclohexyl-1,3-propanediamine, N-cyclohexyl-1,4-butanediamine andN-cyclohexyl-1,5-pentanediamine.
 9. The process of claim 1 wherein thealkaline material is potassium carbonate.
 10. The process of claim 1wherein said polyoxyethylene polyoxypropylene block copolymerantifoaming agent is present in said acid gas scrubbing solution in anamount ranging from about 1 to about 1500 parts per million by weight ofthe acid gas scrubbing solution.
 11. The process of claim 9 wherein theamine activator is N-cyclohexyl-1,3-propanediamine.
 12. The process ofclaim 11 wherein the acid gas scrubbing solution additionally includesan aminoacid cosolvent containing 4-8 carbon atoms.
 13. The process ofclaim 12 wherein the cosolvent is pipecolinic acid.
 14. The process ofclaim 13 wherein the acid gas scrubbing solution contains more than oneantifoaming agent.
 15. In a process for removing CO₂ from gaseous feedscontaining CO₂, which comprises:(1) contacting said gaseous feeds withan aqueous scrubbing solution comprising: (a) an alkaline materialcomprising a basic alkali metal salt or alkali metal hydroxide, (b) anactivator for said alkaline material comprising at least one amineactivator for said alkaline material, (c) water and (d) at least oneantifoaming agent; (2) regenerating said aqueous scrubbing solution atconditions whereby CO₂ is desorbed from said aqueous scrubbing solution;and (3) passing the gas from step (1) into a caustic scrubber whichremoves traces of CO₂ in the gas and functions as a trap for aminescarried into the conduit leading to the scrubber by the treated gas, theimprovement which comprises providing at least one antifoaming agent insaid caustic scrubber comprising polyoxyethylene polyoxypropylene blockcopolymers of the general formula:

    HO(C.sub.2 H.sub.4 O).sub.a (C.sub.3 H.sub.6 O).sub.b (C.sub.2 H.sub.4 O).sub.c H

wherein a and c are integers such that the hydrophile portionrepresented by (C₂ H₄ O) constitutes from about 10 to about 30 weightpercent of the block copolymer and b is an integer such that thehydrophobe represented by (C₃ H₆ O) has a molecular weight from about1000 to about
 2150. 16. The process of claim 15 wherein a and c of saidblock copolymer are integers such that the hydrophile portionrepresented by (C₂ H₄ O) constitutes from about 15 to about 25 weightpercent of the block copolymer and b is an integer such that thehydrophobe represented by (C₃ H₆ O) has a molecular weight ranging fromabout 1200 to about
 2100. 17. The process of claim 16 wherein b of theblock copolymer is such that the hydrophobe portion represented by (C₃H₆ O) has a molecular weight ranging from about 1750 to about
 2050. 18.The process of claim 15 wherein at least one of the amine activators isa sterically hindered amine defined as a compound containing at leastone secondary amine group attached to either a secondary or tertiarycarbon atom or a primary amino group attached to a tertiary carbon atom.19. The process of claim 18 wherein said sterically hindered amine isselected from the group consisting of aminoethers, aminoalcohols, di-and triamines.
 20. The process of claim 18 wherein the stericallyhindered amine is a diamine.
 21. The process of claim 20 wherein thediamine is a cycloaliphatic diamine.
 22. The process of claim 20 whereinthe diamine is selected from the group consisting ofN-cyclohexyl-1,2-ethanediamine, N-cyclohexyl-1,3-propanediamine,N-cyclohexyl-1,4-butanediamine and N-cyclohexyl-1,5-pentanediamine. 23.The process of claim 15 wherein the alkaline material is potassiumcarbonate.
 24. The process of claim 15 wherein said polyoxyethylenepolyoxypropylene block copolymer antifoaming agent is present in saidacid gas scrubbing solution in an amount ranging from about 1 to about1500 parts per million by weight of the acid gas scrubbing solution. 25.The process of claim 23 wherein the amine activator isN-cyclohexyl-1,3-propanediamine.
 26. The process of claim 25 wherein theacid gas scrubbing solution additionally includes an aminoacid cosolventcontaining 4-8 carbon atoms.
 27. The process of claim 26 wherein thecosolvent is pipecolinic acid.
 28. The process of claim 27 wherein theacid gas scrubbing solution contains more than one antifoaming agent.